Differentiation of mono‐oxo and polyoxo and of monomeric and polymeric vanadate, molybdate and tungstate species in metal oxide catalysts by IR and Raman spectroscopy

Abstract

The expected and experimental features of vanadate, molybdate and tungstate species observed in inorganic compounds of different structures are discussed. In particular, fourfold coordinated species (truly tetrahedral and distorted tetrahedral), isolated and polymeric mono-oxo and dioxo metallyl species are discussed. On these bases, some of the data concerning surface species in oxide-supported oxides are also considered. The need to perform both IR and Raman experiments and, if possible, O-18/O-16 exchange experiments is also emphasized. Copyright (C) 2002 John Wiley Sons, Ltd

Molecular Structure of a Supported VO4 Cluster and Its Interfacial Geometry as a Function of the SiO2, Nb2O5, and ZrO2 Support

“…At dehydrated conditions and low loadingsbelow the monomeric monolayer (2.3 VO x /nm 2 ) as defined by Khodakov 7 sthe vanadium oxide species on the surface will be present only as a monomeric VO 4 cluster with one VdO bond. 17,18 EXAFS data analysis showed that for all supports the VO 4 cluster is attached to the surface with only one V-O b -M support bond under the applied measurement conditions. Both the V-O b bond distance and the V‚‚‚ M support distance depend on the structure of the support surface.…”

Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

“…Furthermore, other species (d) and (e) are proposed to be formed under various measurement conditions. The di-oxo structure (a) is commonly rejected based on the results of EXAFS analysis [30][31][32][33] and the absence of a symmetric and anti-symmetric V O stretching band in the vibrational spectra [59][60][61]. The single V O nature of the VO 4 unit has been confirmed with 18 O labeling experiments in conjunction with Raman spectroscopic analysis [60,61].…”

Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors

“…The intensities of the absorption in the OH region in the spectra of the ion-exchanged samples parallel the intensities of the W=O bands. This indicates that the OH coverage of the activated samples is associated with hydroxyls produced by dissociative adsorption of ambient water on coordinatively unsaturated (cus) W=O groups [38]. The OH groups complete the coordination sphere of the W=O species and give rise to Brønsted acid sites [38].…”

“…This indicates that the OH coverage of the activated samples is associated with hydroxyls produced by dissociative adsorption of ambient water on coordinatively unsaturated (cus) W=O groups [38]. The OH groups complete the coordination sphere of the W=O species and give rise to Brønsted acid sites [38]. Since the number of the cus W=O species decreases with the increase in the W coverage, the amount of the W-OH groups decreases as well.…”